Problem: Which of these numbers is prime? ${26,\ 29,\ 55,\ 65,\ 99}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 26 are 1, 2, 13, and 26. The factors of 29 are 1 and 29. The factors of 55 are 1, 5, 11, and 55. The factors of 65 are 1, 5, 13, and 65. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 29 is a prime number.